1. Field of the Invention
The present invention relates to a turbocharged engine and more particularly to a turbocharged engine which has the best supercharging character for a motor vehicle.
2. Description of the Related Art
A conventional turbocharged engine 73 for a motor vehicle, as shown in FIGS. 4, 5, is disclosed in Japanese Utility Model Laid-open No. 59(1984)-534. Here, each one end of an intake passage 71 and an exhaust passage 72 are connected to the engine 73. An air cleaner 74 is connected to the other end of the intake passage 71. A compressor rotor 75 of a turbocharger 70 is inserted in the intake passage 71 and a turbine rotor 76 thereof is inserted in the exhaust passage 72. A shaft 77 connects the compressor rotor 75 and the turbine rotor 76.
In the exhaust passage 72, a bypass passage 78 connects upstream side of the turbine rotor 76 and downstream side thereof. A waste gate valve 79 opens or closes an opening 82 formed between the bypass passage 78 and the upstream side of the turbine rotor 76 in the exhaust passage 72 via a valve 81 according to a signal pressure S. The signal pressure S is supplied from a port 80 formed at a downstream side of the compressor rotor 75 in the intake passage 71.
In the waste gate valve 79, an atmospheric pressure chamber 83 and a signal pressure chamber 84 are formed in the housing and are separated by a diaphragm 85 from each other. One end of the valve 81 is fixed to the diaphragm 85. Springs 86,87 are located in the atmospheric pressure chamber 83 and urge the valve 81 downwardly. The signal pressure S is introduced into the signal pressure chamber 84.
In the above-mentioned turbocharged engine 73, an exhaust gas E flows in the exhaust passage 72 and makes the turbine rotor 76 rotate. So, the compressor rotor 75 is rotated with the turbine rotor 76 via the shaft 77. Intake air N is supercharged to the engine 73 by the compressor rotor 75. One part of the supercharging pressure is introduced as the signal pressure S to the signal pressure chamber 84. The signal pressure S in the signal pressure chamber 84 urges the valve 81 upwardly against the springs 86,87. Thus, if the signal pressure S is over a setting pressure (ex. 0.5.about.0.8[kg/cm]), fluid communication between the exhaust passage 72 and the bypass passage 78 is established. So, one part of the exhaust gas E flows into the bypass passage 78, and the revolution number of the turbine rotor 76 decreases. Consequently, the supercharging pressure also decreases.
In FIG. 6, an ideal supercharging character is shown by the solid line X, and an supercharging character of the turbocharger 70 is shown by the chain line Y. Namely, after the supercharging pressure increases to the point B along the solid line, the supercharging pressure is kept at a fixed pressure. That is to say, from the high pressure region over the point B, the valve 81 opens the opening 82. This is the ideal supercharging character. However, in the turbocharged engine 73, a pressure of the exhaust gas pushes the valve 81 upwardly with the signal pressure S, so for a pressure beyond the point T, the valve opens the opening 82. Thus, a sufficient supercharging pressure is not achieved in the region Z.